Electrodeposition of Silicon in Fluoride Melts: Production of Silicon Films
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There has been considerable interest in electrodeposition of silicon from fluoride melts on a suitable substrate for its application in thin film solar cells. The goal of this work is to produce a high purity silicon films from LiF-KF-K2SiF6 (mol %) that could be suitable for solar grade applications, and to study electrodeposition of silicon in the same melt by performing electrochemical measurements. Cyclic voltammetry was carried out both in pure melt LiF-KF and LiF-KF-K2SiF6 (0.13mol/kg) at 750oC on Ag electrode to study the reduction mechanism of fluorosilicate. The reduction mechanism was found to be mass transport diffusion controlled. The diffusion coefficient was estimated to be 1.1x10-5cm2/s from Randles-Sevcik equation.Chronoamperometry was also carried out in LiF-KF-K2SiF6 (0.13mol/kg) at 750oC on Ag electrode at different cathodic potentials and the current-time response, reduction mechanism of silicate ion was studied. It was also found again that the reduction mechanism of fluorosilicate is diffusion controlled. The diffusion coefficient was calculated to be 4.6x10-4cm2/s using the Cottrell equation. The influence of electrolytic parameters such as temperature, concentration of the electroactive species K2SiF6, and current density on the morphology and purity of the deposits and the current efficiency of the electrolytic process was studied. Effect of temperature and concentration was studied on Ag substrate and current density on both Ag and Si substrate. The influence of the substrate (silver and silicon) was also studied. Before the start of each electrodeposition experiment pre-electrolysis was carried out to remove moisture and reduce impurities concentration to ppm level. The deposits obtained were cleaned in ultrasonic bath to get rid of salt inclusions. It was observed that not all the salt inclusions were completely removed and therefore the current efficiency calculated was described to be apparent (not accurate). A few selected deposits were characterized using scanning electron microscope and energy dispersive spectroscopy, the results are shown below. It was observed that at a temperature of 800oC the deposit was dense, coherent, good adhesion to the silver substrate and with less impurities and salt inclusions but at 550oC the deposit was powdered or dendritic with high content of impurities and salt inclusions. At 5mol% of K2SiF6 the deposit consists of homogeneous structure with less impurities and salt but at 20mol% on the microstructure seems to be elongated and contain high content of impurities and salts. On silicon substrate, at current density 101.5mAcm-2 the deposit was dendritic with no grains, weakly adhered to the silicon substrate and also contains more impurities and salts but at 35.5mAcm-2 the deposit contains large grains with columnar microstructure and less impurities and salts. On silver substrate, at 83.8mAcm-2 the deposit consists of fine microstructure with high content of impurities and salts but at 42.9mAcm-2 the deposit consists of bigger and elongated with high porosity and small content of impurities and salts. For the influence of the substrate, it was observed that on sliver the deposit was insoluble with fine microstructure but on silicon it was soluble and weakly adhered with non uniform microstructure which is powdered and dendritic. The deposit obtained on both silver and silicon contains high content of impurities and salts because of the high current density applied (83.8mAcm-2 on Ag and 80.5mAcm-2 on Si).